A cylinder block used in an automobile engine or the like is ordinarily manufactured by a casting process, but recently, in consideration of achieving high processing precision and shortening of the processing time, such cylinder blocks have also been often manufactured via die cast formation. In addition, in consideration of lightening the weight of the cylinder block, and so forth, cylinder blocks made of an aluminum alloy have also become widespread.
With respect to the shape of the cylinder block, there is an open deck type in which a water jacket serving as a coolant water path formed within the cylinder block is open at a block top face (face combined with the cylinder head: a deck face), and a closed deck type in which the water jacket is not open at the block top face. Of these types, the open deck type does not require a core in the casting process, and therefore is particularly suitable for a die cast cylinder block as described above.
As a conventional example of a die cast open deck type cylinder block, for example, technology as disclosed in PTL 1 has been proposed. In this technology, a water jacket in the vicinity of a head bolt hole where a head bolt for attaching the cylinder head is fastened has a shallowed structure. By adopting such a shallowed structure to reduce the width of the water jacket, so as to bring a cylinder bore and the head bolt hole closer together, the fastening force of the head bolt is easily transmitted around a cylinder bore, so sealing around the cylinder bore is improved, and rigidity of the outer wall and the inner wall (cylinder) of the water jacket is improved.